He began testing designs using tiny globs of metal called "quantum dots" (QDs) sprinkled on a nanoinsulator. For the insulator substrate he chose boron nitride nanotubes, known as BNNTs. For the quantum dots he used gold, an ideal material for making regular, precisely-sized QDs.

The team needed a way to position the dots in atomic space on the nanotube, so they turned to using a laser. Using this method, they were able to positions gold QDs that were a mere 3 nanometers in diameter -- or roughly 1/7th the size of transistors produced at current circuit manufacturing nodes.

An artist's rendering of the nanotube transistor [Image Source: MTU]

Testing the design in collaboration with Oak Ridge National Laboratory (ORNL), they hooked an electrode up to each end of the construct and tested it at room temperature. They observed quantum tunneling -- the hallmark phenomena necessary to construct a non-semiconductor transistor. Electrons "jump" (or tunnel) from one gold QD to the next, as current is applied.

The researchers were able to control the voltage to switch this conduction on and off, forming a transistor. Professor Yap describes, "Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river. The electrons hopped between the gold stepping stones. The stones are so small, you can only get one electron on the stone at a time. Every electron is passing the same way, so the device is always stable."

Past transistors made from materials other than semiconductor typically had to be cooled with liquid helium to operate well; by contrast Professor Yap's design performs well at room temperature.

Currently each nanotube is 1 micron long and 20 nm wide -- making these transistors on par with current designs. But the researchers expect these transistors to scale better than semiconductor sizes as tube diameter and lengths are shrunk. The team already has established methods to deposit aligned substrate "carpets" so now all that remains is developing methods to mass-position the nanodots (as individual laser positioning is prohibitively slow for making the billions of transistors in a modern IC).

Electron micrographs of nanotube "carpets" grown by Prof. Yap's team back in 2011.
[Image Source: MTU]

Professor Yap, who has filed for a patent on the design and manufacturing process, comments, "Theoretically, these tunneling channels can be miniaturized into virtually zero dimension when the distance between electrodes is reduced to a small fraction of a micron."

The best feature of the transistors is that there's no electrons (according to the authors) lost between gold nanodot and no gold nanodot -- a heat generating phenomena known as "leakage". By contrast, leakage is a massive problem for nanoscale silicon-based transistors, limiting clock speeds and circuit density.

In addition to the patent his work was published [abstract] in a peer-reviewed journal article in the Advanced Materials journal from publisher Wiley.

The way they solve it is by moving (through quantum tunneling) only one electron from gold dot to gold dot, instead of pushing a current of electrons, like semiconductors do.

When you juice up the electrode on the source side, you're not pushing the electrode's current through the path of nanodots. You are determining how many hops a single electron will reach, so the very concept of leakage does not apply.